Bistable cholesteric liquid crystal displays were introduced in the early 1990's (see U.S. Pat. Nos. 5,437,811 and 5,453,863). Their zero-power image retention and sunlight readability led to their integration into numerous signs and battery-powered applications as reviewed in “Cholesteric Liquid Crystals for Flexible Displays” in Flexible Flat Panel Displays, Ed. G. Crawford, (John Wiley & Sons, 2005) J. W. Doane and A. Khan, Chapter 17. The technology is best suited for reflective color images. In the cholesteric display technology, multiple-color and full-color displays are preferably produced by stacking multiple cholesteric liquid crystal layers with each tuned to reflect a different wavelength, typically red, green, and blue (see U.S. Pat. No. 6,654,080). These three colors are additively mixed to achieve up to eight colors. Images with more colors are possible because the technology is amenable to grayscale. That is, the reflective brightness of each color can be electronically adjusted to any desired level between the display's maximum and minimum brightness. Each level of brightness is referred to as a gray level. The total number of colors depends upon the number of gray levels one can choose for each color layer. High resolution displays with as many as 4096 colors have been produced.
Commercial bistable cholesteric displays of the prior art display digital images and as such are made using of a matrix of pixels with each of the pixels having a small area. The resolution of the display depends upon the number of pixels and size of the display. Typical pixel sizes are substantially less than one square millimeter. These displays are typically manufactured on glass substrates. Recent progress has been made in commercializing displays built on flexible plastic substrates rather than glass. The new flexible displays are manufactured with a simple lamination process, and may be cut into interesting shapes after assembly. Of significance, these displays are very thin since thin plastic sheet material as thin as 12.5 microns can be used for the substrates making possible a display with the over all thickness less that 60 microns. Using cholesteric liquid crystals dispersed as emulsified droplets has made possible even thinner displays since all the materials of the display including the electrodes, substrates and cholesteric dispersion can be coated in thin layers.
Such developments suggest a display film that can be electronically switched from one color to another color that can be laminated to flat surfaces and even made to conform to curved surfaces in the form of a skin. Consumers frequently identify color as a necessity for several types of products, such as; clothing, accessories, hand held electronics such cell phones, personally worn electronics, medical indicators, and decorative items. The color on these items is defined on the product when purchased. Conventionally, it has not been possible to electronically change the color of these items after the initial purchase. Thin flexible displays for changing the color of articles, for example, an electrochromic layer or a cholesteric display skin for changing the color of cell phones, have been described in the patent literature but such devices have not been successfully implemented (Published Patent Application No. 2008/0074383 and U.S. Pat. No. 7,142,190). Such cholesteric display skins would suffer from a problem of gray scale discontinuity discussed below. Other products incorporate a color change indicator for either a sensorial signal to indicate the product is properly working or to indicate the user's attention is required. Several color indicator products exist such as battery testers (U.S. Pat. No. 7,188,996) and self expiring security badges (U.S. Pat. No. 6,752,430).
Cholesteric display films have not been suitable for electronic skin applications with tunable uniform colors because uniform gray levels have not been possible in areas around one square centimeter and larger. In areas of such size, the inventors have noticed that levels of gray become very non-uniform or blotchy in appearance. The reason for this is not completely understood but it is believed by the inventors to be a result of several possible causes such as: non-uniform cell gap thickness (varying distance between electrodes) and non-uniform conductivity of transparent electrodes. Such features have not been a problem in typical cholesteric matrix displays because the pixels are so very small that gray levels appear uniform on the scale of a pixel and image content makes the non-uniformity hard to detect across many pixels. Furthermore, nearly all commercial displays have been driven in a binary (on/off) mode not utilizing shades of gray.